U.S. patent application number 12/290705 was filed with the patent office on 2009-08-27 for polarization-maintaining optical coupler and method of making the same.
Invention is credited to Yong Huang.
Application Number | 20090214152 12/290705 |
Document ID | / |
Family ID | 40998392 |
Filed Date | 2009-08-27 |
United States Patent
Application |
20090214152 |
Kind Code |
A1 |
Huang; Yong |
August 27, 2009 |
Polarization-maintaining optical coupler and method of making the
same
Abstract
Embodiments of the present invention provide an optical coupler.
The optical coupler includes a fused optical coupling region made
of part of a first and a second single-mode (SM) fiber, with a
portion of the second SM fiber being a pigtail fiber on a first
side of the fused optical coupling region; and a first and a second
polarization-maintaining (PM) fiber on the first side and a second
side of the fused optical coupling region respectively. Methods of
making the optical coupler and system that utilizes the optical
coupler are also provided.
Inventors: |
Huang; Yong; (Milpitas,
CA) |
Correspondence
Address: |
Yong Huang
1079 Sandalwood LN
Milpitas
CA
95035
US
|
Family ID: |
40998392 |
Appl. No.: |
12/290705 |
Filed: |
November 3, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12075517 |
Mar 11, 2008 |
7492977 |
|
|
12290705 |
|
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Current U.S.
Class: |
385/11 |
Current CPC
Class: |
G02B 6/2843 20130101;
G01R 15/247 20130101; G01R 15/246 20130101; G02B 6/2835
20130101 |
Class at
Publication: |
385/11 |
International
Class: |
G02B 6/00 20060101
G02B006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 21, 2008 |
CN |
200810043138.4 |
Claims
1. An optical coupler, comprising: a fused optical coupling region
made of part of at least a first and a second single-mode (SM)
fiber, with a portion of said second SM fiber being a pigtail fiber
on a first side of said fused optical coupling region; and a first
and a second polarization-maintaining (PM) fiber on said first side
and a second side of said fused optical coupling region
respectively.
2. The optical coupler of claim 1, wherein said first PM fiber is
spliced to said first SM fiber at a location sufficiently close to
said first side of said fused optical coupling region, and said
second PM fiber is spliced to said second SM fiber at a location
sufficiently close to said second side of said fused optical
coupling region.
3. The optical coupler of claim 1, wherein said first PM fiber is
spliced to said first SM fiber at a location sufficiently close to
said first side of said fused optical coupling region, and said
second PM fiber is spliced to said first SM fiber at a location
sufficiently close to said second side of said fused optical
coupling region.
4. The optical coupler of claim 1, wherein said first PM fiber is
spliced to said first SM fiber at less than about 10 mm away from
said first side of said fused optical coupling region, and said
second PM fiber is spliced to said second SM fiber at less than
about 10 mm away from said second side of said fused optical
coupling region.
5. The optical coupler of claim 1, wherein said first PM fiber is
spliced to said first SM fiber at less than about 10 mm away from
said first side of said fused optical coupling region, and said
second PM fiber is spliced to said first SM fiber at less than
about 10 mm away from said second side of said fused optical
coupling region.
6. The optical coupler of claim 1, wherein said portion of said
second SM fiber has a length of at least 150 mm long.
7. An optical coupler, comprising: a fused optical coupling region
being made of part of at least a first, a second, and a third
single-mode (SM) fiber; a first polarization-maintaining (PM) fiber
being spliced to said first SM fiber at a location sufficiently
close to a first side of said fused optical coupling region; and at
least a second PM fiber being spliced to said second SM fiber at a
location sufficiently close to a second side of said fused optical
coupling region.
8. The optical coupler of claim 7, wherein said part of said first,
second, and third SM fibers in said fused optical coupling region
are arranged either linearly in a row or in a tightly spaced
two-layer stack.
9. The optical coupler of claim 7, further comprising a third PM
fiber being spliced to said third SM fiber at a location
sufficiently close to said second side of said fused optical
coupling region, wherein said first, second, and third PM fibers
are spliced to said first, second, and third SM fibers,
respectively, at less than about 10 mm away from said fused optical
coupling region.
10. The optical coupler of claim 9, further comprising a fourth PM
fiber being spliced to said first SM fiber at a location
sufficiently close to said second side of said fused optical
coupling region, thereby said optical coupler being at least a
one-by-three optical coupler.
11. The optical coupler of claim 10, wherein said fused optical
coupling region being made of said part of said first, second,
third SM fibers and part of a fourth SM fiber, further comprising a
fifth PM fiber being spliced to said fourth SM fiber at a location
sufficiently close to said second side of said fused optical
coupling region, thereby said optical coupler being at least a
one-by-four optical coupler.
12. The optical coupler of claim 11, wherein said fused optical
coupling region being made of said part of said first, second,
third, fourth SM fibers and part of a fifth SM fiber, further
comprising a sixth PM fiber being spliced to said fifth SM fiber at
a location sufficiently close to said second side of said fused
optical coupling region, thereby said optical coupler being at
least a one-by-five optical coupler.
13. The optical coupler of claim 7, wherein said fused optical
coupling region being made of a plurality of SM fibers, further
comprising M additional fibers on said first side of said fused
optical coupling region, and N additional fibers on said second
side of said fused optical coupling region, thereby said optical
coupler being a M-by-N optical coupler, wherein said M and said N
being integers larger than three, and said plurality being larger
than three and being at least equal to said M or said N.
14. The optical coupler of claim 13, wherein said M and N
additional fibers are selected from a group consisting of SM fiber,
PM fiber and any combination thereof, and said plurality of SM
fibers are selected from a group consisting of Corning SMF28 fiber,
Corning HI1060Flex fiber, OFS980 fiber, Chang Fei G652 fiber, and
any combination thereof.
15. A method of making an optical coupler, comprising: providing a
plurality of segment-type fibers, said segment-type fibers having
at least a first segment of polarization-maintaining (PM) fiber and
a second segment of single-mode (SM) fiber; arranging said
plurality of segment-type fibers in a bundle, said bundle having at
least one said first segment of PM fiber and one said second
segment of SM fiber at each side of said bundle, and having a
common section of tightly spaced said second segment of SM fibers;
and fusing and drawing said common section into a fused optical
coupling region.
16. The method of claim 15, wherein providing said plurality of
segment-type fibers comprises creating said segment-type fibers by
splicing said first segment of PM fiber to said second segment of
SM fiber.
17. The method of claim 16, wherein splicing said first segment of
PM fiber to said second segment of SM fiber comprises selecting
said first segment of PM fiber and said second segment of SM fiber
such that a difference between mode-field diameters of said first
segment of PM fiber and said second segment of SM fiber is less
than about 20% for a predetermined wavelength range of light.
18. The method of claim 15, wherein arranging said plurality of
segment-type fibers comprises arranging said second segment of SM
fibers of said segment-type fibers in a multi-layer stack.
19. The method of claim 15, wherein arranging said plurality of
segment-type fibers comprises aligning fast and slow polarization
directions of said first segment of PM fibers of said segment-type
fibers in a same orientation.
20. The method of claim 15, wherein said first segment of PM fiber
being spliced to said second segment of SM fiber at a connection
point, wherein fusing and drawing said common section comprises
making said connection point less than about 10 mm away from said
fused optical coupling region.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation-in-part (CIP) of
U.S. patent application Ser. No. 12/075,517, filed Mar. 11, 2008,
entitled "All-Fiber Current Sensor", and claims the benefit of
priority of Chinese patent application serial number
200810043138.4, filed Feb. 21, 2008 with the State Intellectual
Property Office (SIPO) of the People's Republic of China, the
content of which are incorporated herein by reference in their
entireties.
FIELD OF THE INVENTION
[0002] The present invention relates to optical couplers and
devices that make use thereof. In particular, it relates to
polarization-maintaining optical couplers and devices and systems
that incorporate and make use of the polarization-maintaining
optical couplers.
BACKGROUND
[0003] Optical couplers having properties of maintaining
polarization directions of light are widely used in applications
such as, for example, devices and/or systems that are designed for
measuring and/or sensing electric current. Optical couplers used in
electric current sensors are generally made of optical fibers, and
up until recently made of polarization-maintaining (PM) fibers.
Optical couplers with polarization-maintaining property are
referred to, hereinafter, as PM couplers.
[0004] As is known in the art, PM fibers rely upon externally
induced stresses to maintain polarization directions of light
propagating therein. Therefore, performance of PM couplers made of
PM fibers are generally sensitive to both temperature and
polarization state of input light. This makes these PM couplers not
only difficult to make but the manufacturing of these PM couplers
demand high degree of diligence and care during assembly in order
to achieve certain expected quality, so far the combination of
which has lead to low yield in production, poor quality, and
certain reliability issues of the final product. Nevertheless, PM
couplers, and other similar optical coupling devices, are finding
more and more applications in a variety of optical systems, large
or small, in addition to electric current sensors.
[0005] An electric current sensor, or sensing device, or system
that is optical fiber based works in the principle of well-known
Faraday Effect. It is generally known that an electric current
propagating inside a wire or conductor will induce a magnetic field
around the wire or conductor. Now, assume that a light is traveling
inside an optical fiber that is wound around a current-carrying
wire or conductor. Through Faraday Effect, the magnetic field
induced by the current may cause polarization direction of the
light to rotate. According to Faraday's law, the degree of rotation
will be directly proportional to the magnitude of the magnetic
field, thus directly proportional to the magnitude of the electric
current carried by the conductor or wire, as well as to the total
length of optical path traversed by the light. Therefore, by
injecting a light of pre-defined for example linear polarization
state into a fiber placed in the sensing region (i.e., magnetic
field region) of a wire or conductor and by measuring, and
subsequently analyzing, changes in the polarization state of light
exiting from the sensing region in the fiber, the amount of
electric current carried by the wire or conductor may be
determined.
[0006] In a fiber-optic based current sensor as described above,
the magnitude of rotation of polarization direction of light is
usually small. For that reason, a fiber-optic based current sensor
usually employs the configuration of a Sagnac interferometer using
an optical coupler for improving sensitivity in the detection of
polarization direction rotation. However, since a conventional
optical coupler made of polarization-maintaining fibers is not
capable of properly maintaining polarization directions of light
propagating therein, sensors employing conventional optical
couplers frequently exhibit unstable device performance due to, at
least partially, uncertainty of polarization state of light inside,
and have limited value in practice.
[0007] In view of the above, there is a need in the art to develop
solutions that will address above-mentioned shortcomings of
existing fiber-optic based electric current sensors; and in
particular, there is an urgent need to develop reliable
polarization-maintaining optical couplers with improved yield and
generally acceptable device performance.
SUMMARY OF EMBODIMENTS
[0008] Embodiments of the present invention provide an optical
coupler. The optical coupler includes a fused optical coupling
region made of part of at least a first and a second single-mode
(SM) fiber, with a portion of the second SM fiber being a pigtail
fiber on a first side of the fused optical coupling region; and a
first and a second polarization-maintaining (PM) fiber on the first
side and a second side of the fused optical coupling region
respectively. In one embodiment, the portion of the second SM fiber
has a length of at least 150 mm long.
[0009] According to one embodiment, the first PM fiber is spliced
to the first SM fiber at a location sufficiently close to,
preferably less than 10 mm away from, the first side of the fused
optical coupling region, and the second PM fiber is spliced to the
second SM fiber at a location sufficiently close to, preferably
less than 10 mm away from, the second side of the fused optical
coupling region.
[0010] According to another embodiment, the first PM fiber is
spliced to the first SM fiber at a location sufficiently close to,
preferably less than 10 mm away from, the first side of the fused
optical coupling region, and the second PM fiber is spliced to the
first SM fiber at a location sufficiently close to, preferably less
than 10 mm away from, the second side of the fused optical coupling
region.
[0011] Embodiments of the present invention also provide an optical
coupler, which includes a fused optical coupling region made of
part of at least a first, a second, and a third single-mode (SM)
fiber; a first polarization-maintaining (PM) fiber being spliced to
the first SM fiber at a location sufficiently close to a first side
of the fused optical coupling region; and a second PM fiber being
spliced to the second SM fiber at a location sufficiently close to
a second side of the fused optical coupling region, wherein the
part of the first, second, and third SM fibers in the fused optical
coupling region are arranged either linearly in a row or in a
tightly spaced multi-layer, such as two-layer, stack.
[0012] According to one embodiment, the optical coupler may include
a third PM fiber being spliced to the third SM fiber at a location
sufficiently close to the second side of the fused optical coupling
region, wherein the first, second, and third PM fibers are spliced
to the first, second, and third SM fibers respectively at less than
about 10 mm away from the fused optical coupling region.
[0013] According to one embodiment, the optical coupler may include
a fourth PM fiber being spliced to the first SM fiber at a location
sufficiently close to the second side of the fused optical coupling
region, thereby being at least a one-by-three optical coupler.
[0014] According to one embodiment, the fused optical coupling
region may be made of part of the first, second, third SM fibers
and at least part of a fourth SM fiber, and the optical coupler,
being at least a one-by-four optical coupler, may further include a
fifth PM fiber being spliced to the fourth SM fiber at a location
sufficiently close to the second side of the fused optical coupling
region. According to yet another embodiment, the fused optical
coupling region may be made of part of the first, second, third,
fourth SM fibers and at least part of a fifth SM fiber, and the
optical coupler, being at least a one-by-five optical coupler, may
further include a sixth PM fiber being spliced to the fifth SM
fiber at a location sufficiently close to the second side of the
fused optical coupling region.
[0015] Embodiments of the present invention further provide an
optical coupler that includes a fused optical coupling region being
made of multiple SM fibers; M additional fibers on the first side
of the fused optical coupling region; and N additional fibers on
the second side of the fused optical coupling region, thereby the
optical coupler being a M-by-N optical coupler, wherein the
multiple being larger than three (3) and being larger or at least
equal to the number M and/or the number N.
[0016] Embodiments of the present invention further provide a
method of making an optical coupler. The method includes providing
a plurality of segment-type fibers, the segment-type fibers having
a first segment of polarization-maintaining (PM) fiber and a second
segment of single-mode (SM) fiber; arranging the plurality of
segment-type fibers in a bundle, the bundle having at least one the
first segment of PM fiber and one the second segment of SM fiber at
each side of the bundle, and having a common section of tightly
spaced the second segment of SM fibers; and fusing and drawing the
common section into a fused optical coupling region.
[0017] According to one embodiment, providing the plurality of
segment-type fibers includes forming the segment-type fibers by
splicing together the first segment of PM fiber with the second
segment of SM fiber.
[0018] According to another embodiment, splicing the first segment
of PM fiber to the second segment of SM fiber includes selecting
the first segment of PM fiber and the second segment of SM fiber
such that a difference between mode-field diameters of the first
segment of PM fiber and the second segment of SM fiber is less than
about 20%, for a predetermined wavelength range of light.
[0019] According to yet another embodiment, arranging the plurality
of segment-type fibers includes arranging the second segment of SM
fibers of the segment-type fibers in a multi-layer stack.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The present invention will be understood and appreciated
more fully from the following detailed description of embodiments
of the invention, taken in conjunction with the accompanying
drawings of which:
[0021] FIG. 1 is a simplified illustration of a conventional 3-by-3
polarization-maintaining coupler made of polarization-maintaining
fibers as is known in the art;
[0022] FIGS. 2a and 2b are demonstrative illustrations of a 3-by-3
polarization-maintaining coupler according to one embodiment of the
present invention;
[0023] FIGS. 3a and 3b are demonstrative illustrations of
segment-type fibers according to some embodiments of the present
invention;
[0024] FIGS. 4a and 4b are demonstrative illustrations of 3-by-3
polarization-maintaining couplers according to some embodiments of
the present invention;
[0025] FIGS. 5a and 5b are demonstrative illustrations of 2-by-2
polarization-maintaining couplers according to some other
embodiments of the present invention;
[0026] FIG. 6 is a demonstrative illustration of a 5-by-5
polarization-maintaining coupler according to yet some other
embodiment of the present invention;
[0027] FIG. 7 is a simplified flowchart illustration of method of
fabricating a polarization-maintaining coupler according to one
embodiment of the present invention;
[0028] FIG. 8 is a simplified block diagram illustration of an
all-fiber electric current sensor applying a 3-by-3
polarization-maintaining coupler according to one embodiment of the
present invention; and
[0029] FIG. 9 is a simplified block diagram illustration of an
all-fiber electric current sensor applying a 3-by-3
polarization-maintaining coupler according to another embodiment of
the present invention.
[0030] It will be appreciated that for simplicity and clarity of
illustration, elements shown in the figures have not necessarily
been drawn to scale. For example, the dimensions of some of the
elements may be exaggerated relative to other elements for
clarity.
DETAILED DESCRIPTION OF EMBODIMENTS
[0031] In the following detailed description, numerous specific
details are set forth in order to provide a thorough understanding
of embodiments of the present invention. However it will be
understood by those of ordinary skill in the art that embodiments
of the present invention may be practiced without these specific
details. In other instances, well-known methods and procedures have
not been described in detail so as not to obscure description of
embodiments of the invention.
[0032] Some portions of the detailed description in the following
are presented in terms of algorithms and symbolic representations
of operations on electrical and/or electronic signals, and optical
signals. These algorithmic descriptions and representations may be
the techniques used by those skilled in the electrical and
electronic engineering and optical communication arts to convey the
substance of their work to others skilled in the art.
[0033] An algorithm is here, and generally, considered to be a
self-consistent sequence of acts or operations leading to a desired
result. These include physical manipulations of physical
quantities. Usually, though not necessarily, these quantities take
the form of electrical or electronic or optical signals capable of
being stored, transferred, combined, compared, converted, and
otherwise manipulated. It has proven convenient at times,
principally for reasons of common usage, to refer to these signals
as bits, values, elements, symbols, characters, terms, numbers or
the like. It should be understood, however, that all of these and
similar terms are to be associated with the appropriate physical
quantities and are merely convenient labels applied to these
quantities.
[0034] In the following description, various figures, diagrams,
flowcharts, models, and descriptions are presented as different
means to effectively convey the substances and illustrate different
embodiments of the invention that are proposed in this application.
It shall be understood by those skilled in the art that they are
provided merely as exemplary and/or demonstrative samples, and
shall not be constructed as limitation to the invention.
[0035] FIG. 1 is a simplified illustration of a conventional 3-by-3
polarization-maintaining coupler made of polarization-maintaining
fibers as is known in the art. In making 3-by-3 PM coupler 100,
generally three traditional PM fibers 110, 120, and 130 are first
linearly aligned, in a closely spaced fashion one next to the
other, as being illustrated in cross-sections A-A', B-B', and C-C'
of FIG. 1. Next, a section of the three closely spaced PM fibers
110, 120, and 130, at around cross-section B-B' in the middle, is
fused and drawn into an optical coupling region as is known in the
art, thus forming 3-by-3 PM coupler 100.
[0036] Here, a person skilled in the art shall appreciate that FIG.
1 as well as other drawings throughout this application are for
illustration purpose only. For example, spaces or small gaps in
between fibers shown in the drawings are only for better
illustration of each individual fiber. The fibers are actually
closely spaced to be in contact at the side surface one next to the
other, as being illustrated at the cross-sections of various
locations. In addition, the cross-sections are shown before the
optical fibers is fused and drawn into an optical coupling region.
The process of fusing and drawing will in general produce a
cross-section that is slightly different from that of cross-section
B-B', for example, with generally closer distance between the fiber
cores, and a single common fused cladding covering all the fiber
cores, as is well know in the art.
[0037] FIGS. 2a and 2b are demonstrative illustrations of a 3-by-3
polarization-maintaining coupler according to one embodiment of the
present invention. PM coupler 200 may be made of three segment-type
(ST) fibers 210, 220, and 230, which may be made of segments of
single-mode fiber and polarization-maintaining fiber according to
some embodiments of the present invention as described below in
detail with reference to FIGS. 3a and 3b. In addition, PM coupler
200 may be a 3-by-3 PM coupler and have an optical coupling region
made through fusing and drawing a middle section 201 of
conventional single-mode (SM) fibers of the three segment-type
fibers. The fused coupling region may have a cross-section area as
illustrated in FIG. 2b, wherein the three fiber cores are spaced
closer than they are, as being illustrated at cross-section B-B' of
FIG. 2a before the fusing and drawing process, with a common fused
cladding covering all three fiber cores.
[0038] Here, it is worth noting that the term N-by-M PM coupler
means that the PM coupler has maximum N input/output ports on one
side and M input/output ports on the other side. In practice, the
PM coupler may be used for any number of input and/or output ports
as desired, for example as an N-by-1 PM coupler, a 1-by-M PM
coupler, or a 2-by-3 PM coupler, up to the maximum available ports
on each side. Ports that are not used may be properly terminated or
may have been designed to have little coupling effect.
[0039] Reference is briefly made to FIGS. 3a and 3b, which are
demonstrative illustrations of segment-type (ST) fibers according
to some embodiments of the present invention. For example,
segment-type fiber 310 may be made of a first segment (piece) of
polarization-maintaining fiber (including sections 311 and 312) and
a second segment (piece) of single-mode fiber (including sections
313 and 314), the two of which are fusion-spliced together at a
point 319. In FIG. 3a, section 312 (of length L1) of PM fiber is
illustrated to be slightly thinner than section 311 to show that
covering jacket and possible buffer layers of the PM fiber may be
removed for splicing purpose. Otherwise, sections 311 and 312 are a
same piece of PM fiber. Similarly, section 313 (of length L2) of SM
fiber is drawn to be slightly thinner than section 314 to
illustrate that covering jacket and possible buffer layers of the
SM fiber may be removed for splicing purpose and otherwise sections
313 and 314 are a same piece of SM fiber.
[0040] Similarly, fiber 320 of a three-segment type may be made of
a first segment or piece of polarization-maintaining fiber
(including sections 321 and 322 of length L3), a second segment or
piece of single-mode fiber 323 of length L4, and a third segment or
piece of polarization-maintaining fiber (including sections 324 of
length L5 and 325). As being demonstratively illustrated in FIG.
3b, the first segment of PM fiber is fusion-spliced with the second
segment of SM fiber at a point 328, and the second segment of SM
fiber is fusion-spliced, at the other end, with the third segment
of PM fiber at a point 329.
[0041] As is illustrated in the cross-section of FIG. 3a, PM fiber
312 has a core 301, and usually two stress rods 302 which induce
birefringence into core 301 of fiber 312. Generally, birefringence
in a PM fiber creates two orthogonal polarization directions or
axes, one fast and one slow, for light-wave propagating therein,
thus provides polarization-maintaining property as is well known in
the art. Examples of polarization-maintaining fibers may include,
for example, 1550 nm Panda fiber or any other suitable current or
future available PM fibers. Examples of single-mode fibers may
include, for example, Corning SMF28 fiber, Corning HI1060Flex
fiber, OFS980 fiber, Chang Fei G652 fiber, and/or any other
suitable current or future available SM fibers.
[0042] Embodiments of the present invention also provide a method
for preparing segment-type fibers such as ST fiber 310 and/or 320.
According to one embodiment, the method may include providing a
piece of PM fiber (e.g., 311 and 312) of a length of, for example,
at least 150 mm long and removing the covering jacket of segment
312 in a length L1 of, for example, from around 2 mm to around 15
mm and preferably around 5 mm to 10 mm. Next, a relatively smooth
and substantially perpendicular end of PM fiber 312 may be
prepared, for splicing purpose, by a blade or a cleaver or any
other suitable means as is known now or being developed in future.
Embodiment of the method may also include providing a piece of SM
fiber (e.g., 313 and 314) of a length of, for example, at least 150
mm or 200 mm long and removing the covering jacket of segment 313
in a length L2 of, for example, from around 13 mm to around 55 mm
and preferably around 20 mm to 25 mm. Here, a person skilled in the
art will appreciate that length L1 and length L2 may be longer or
shorter than those suggested above. In principle, length L2 of SM
fiber 313 shall be sufficiently long so that it may be drawn into
an optical coupling region later, while length L1 of PM fiber 312
shall be sufficient short (in order to make devices made of ST
fiber 310 compact, as described below) but long enough such that it
may be spliced to SM fiber 313 without difficulty. Generally, in
order to reduce coupling loss from PM fiber 312 to SM fiber 313, or
vise versus, fibers 312 and 313 shall be selected to have
approximately same or close mode-field diameters. For example,
according to one embodiment, a difference less than 20% in
mode-field diameter between fibers 312 and 313 is preferred in
order to achieve certain nominally acceptable coupling loss.
Difference in mode-field diameter may be over a pre-determined
wavelength range of, for example, 1510 nm to 1560 nm or 950 nm to
1600 nm, larger or smaller depending on the actual application of
the PM coupler. According to another embodiment, fibers with
mode-field diameter difference larger than 20% may be used as well
depending on actual application needs or system loss requirement.
Next, embodiment of the method includes fusion-splicing PM fiber
312 with SM fiber 313 at point 319 to make ST fiber 310.
[0043] Similarly, ST fiber 320 shown in FIG. 3b may be made by
fusion-splicing PM fiber 322 to SM fiber 323 with a length of L3
being approximately the same as L1 and a length of L4 being
approximately the same as L2, and then by splicing SM fiber 323
with PM fiber 324 with a length of L5 being approximately the same
as L3.
[0044] Reference is made back to FIG. 2a. Segment-type (ST) fibers
210, 220, and 230 may be arranged to be next to each other
linearly, and in contact with each other on side surface, in a
common plane to form a fiber bundle, as is shown in cross-section
A-A', B-B', and C-C' at different locations. At least at one end of
the fiber bundle there is at least one PM fiber segment of the ST
fibers. For example, there is PM fiber segment 221 at the left side
end and there are PM fiber segments 214 and 234 at the right side
end of the fiber bundle. The three segment-type fibers meet in a
common region 201, where they are all made of SM fibers.
[0045] The common region 201 of single-mode fibers are then drawn
into an optical coupling region through, for example, a technique
commonly known as fusion-drawing. According to one embodiment of
the present invention, the optical coupling region may be made
sufficient close to the connection points (splicing points) between
PM fiber and SM fiber. For example, connection point 241 may be
made sufficiently close to end point 251 of common region (or
coupling region) 201, less than 20 mm and preferably less than 10
mm. Similarly connection point 242 may be arranged to be close
enough to end point 252 of common region (or coupling region) 201,
less than 20 mm and preferably less than 10 mm. Generally, the
closer the splicing or connection point to the coupling region, the
more stable the performance of coupler will be. However, certain
tradeoff may be made between coupler performance and the ease of
manufacturing since the closer the splicing point to the coupling
region, in some instances strength of the splicing may be
compromised.
[0046] PM coupler 200 may work like the following. An optical
signal may be launched into, for example, PM fiber 221 and may
propagate to SM fiber 224 that may be directly connected to PM
fiber 221. The optical signal may also be coupled to PM fiber 214
and/or 234 via optical coupling region 201. Alternatively, an
optical signal launched into, for example, SM fiber 224 may
propagate to PM fiber 221, or be coupled to SM fiber 211 and/or 231
via optical coupling region 201. The amount of power propagating to
a directly connected fiber or being coupled to a closely placed
neighboring fiber may depend upon the length of coupling region
201, and how tightly the three SM fibers are fused together.
[0047] FIGS. 4a and 4b are demonstrative illustrations of 3-by-3
polarization-maintaining couplers according to some embodiments of
the present invention. PM coupler 401 may be made of fibers 410,
420, and 430 that are segment-type (ST) fibers, that is, at least
one segment of PM fiber and one segment of SM fiber as described
above. Fibers 410, 420, and 430 may have a common optical coupling
region 409 made of fused single-mode fibers which are part of ST
fibers 410, 420, and 430. Comparing with PM coupler 200 in FIG. 2,
ST fibers 410, 420, and 430 in PM coupler 401 may be arranged in a
stacked fashion, at least in the fused optical coupling region 409.
For example, instead of being placed and aligned in a common plane
as in FIG. 2, one ST fiber 420 may be laid on top of other two ST
fibers 410 and 430, in a tightly close fashion. According to one
embodiment, PM fiber segments of ST fibers 410, 420, and 430 may be
arranged in such a way that their fast and slow polarization axes
are oriented in substantially the same direction. For example, fast
axes of polarization direction may be oriented in the direction of
optical coupling or orthogonal to the optical coupling direction.
However, a person skilled in the art will appreciate that present
invention may not be limited in this respect. For example,
depending on actual optical coupling needs, direction of
polarization axes of the PM fibers may be arranged in different
orientations, and in some instance slow polarization axes of one or
more PM fibers may be oriented with fast polarization axes of one
or more other PM fibers.
[0048] According to one embodiment of the present invention,
single-mode fiber segment of one ST fiber may be different from
single-mode fiber segment of another ST fiber, in terms of core
size, cut-off wavelength, and/or dispersion or dispersion slope,
etc. When ST fibers with different single-mode fibers are used in
making a PM coupler through their fused optical coupling region of
single-mode fibers, different and preferable optical coupling
properties may be obtained. For example, PM couplers with wide
coupling bandwidth may be obtained through this approach, which may
be suitable for accommodating broadband optical signals; optical
signals of multiple wavelengths; and in certain applications, WDM
(wavelength-division-multiplexing) signals.
[0049] Since single-mode fibers are used in forming the fused
optical coupling region, PM coupler 401, and other PM couplers made
in accordance with embodiments of the present invention, may have
the advantages of a regular single-mode optical coupler such as low
polarization dependent loss (PDL) and low temperature sensitivity
of polarization extinction ratio. On the other hand, PM couplers
made in accordance with embodiments of the present invention
provide the performance of maintaining polarization directions of
input and output lights, but without the reliability and/or
stability issues that are often encountered in optical couplers
made of traditional polarization-maintaining fibers. In FIG. 4b, PM
coupler 402 may be made of one three-segment ST fiber 440
(including 441 and 444) and two two-segment ST fibers 450
(including 451 and 454) and 460 (including 461 and 464). ST fibers
440, 450, and 460 are fused together in their SM fiber sections
which form an optical coupling region among the ST fibers.
[0050] A person skilled in the art will also appreciate that the
above 3-by-3 PM coupler may be used, depending on actual
requirement for the number of input and output ports, as 1-by-2,
1-by-3, 2-by-2, or 2-by-3 optical couplers by selecting the desired
number of input and output ports of either PM or SM fibers. The
above 3-by-3 PM coupler may also be used as a regular optical
coupler in situations wherein maintaining polarization directions
of input and/or output light may not be essential or necessary.
[0051] FIGS. 5a and 5b are demonstrative illustrations of 2-by-2
polarization-maintaining couplers according to some other
embodiments of the present invention. PM coupler 501 may be made of
two ST fibers 510 and 520 arranged in such a way that they have a
common fused coupling region made of single-mode fibers, and each
side of PM coupler 501 has one PM fiber and one SM fiber. PM
coupler 502 may be made of two ST fibers 530 and 540, one of which
may be a three-segment ST fiber 530 and the other a two-segment ST
fiber 540. It shall be noted that embodiments of the present
invention may not be limited to either 2-by-2 or 3-by-3 PM couplers
or method of making the same. For example, embodiments of the
present invention may be applied in making M-by-N optical couplers
where M and N may be any suitable integer numbers such as, for
example, 2, 3, 4, and 5.
[0052] For example, FIG. 6 is a demonstrative illustration of a
5-by-5 polarization-maintaining coupler according to yet some other
embodiment of the present invention. In FIG. 6, PM coupler 601 has
five ST fibers, such as ST fibers 610 and 630, arranged in a linear
fashion. Furthermore, it is to be appreciated that PM couplers
according to embodiments of the present invention may be made of a
combination of ST fibers and regular SM fibers. For example, a
5-by-5 PM coupler may be made of two ST fibers and three regular SM
fibers (not shown), with each side of the coupler has at least one
segment of PM fiber.
[0053] FIG. 7 is a simplified flowchart illustration of method of
fabricating a polarization-maintaining coupler according to one
embodiment of the present invention. More specifically, one
embodiment of the method may include steps of preparing ends of
single mode fibers and polarization maintaining fibers for splicing
(701), and making fibers of segment-type (702) by, for example,
fusion-splicing together pigtails of single-mode fibers with
polarization-maintaining fibers. With the segment-type fibers, one
embodiment of the method may include arranging the pre-prepared
segment-type fibers in a bundle, either in a single role or in a
tightly stacked multi-layer fashion (703). The arrangement has, at
each side of the fiber bundle, at least one single-mode fiber
pigtail and at-least one polarization-maintaining fiber pigtail.
According to one embodiment, the method ensures that the fiber
bundle has a middle section of single-mode fibers from SM fiber
segment of the ST fibers (704). Embodiment of the method also
ensures that the splicing point of SM fiber and PM fiber is
arranged to be sufficiently close to the middle section of SM
fibers (705). Furthermore, the method may include steps of fusing
and drawing the middle section of SM fibers into an optical
coupling region (706) thus forming a PM coupler.
[0054] FIG. 8 is a simplified block diagram illustration of an
all-fiber electric current sensor applying a 3-by-3
polarization-maintaining coupler according to one embodiment of the
present invention. Electric current sensor 800 may include a PM
coupler 801, which may be made in accordance with embodiments of
the present invention as described above, and therefore may be able
to maintain polarization state of light propagating therein.
Additionally, PM coupler 801 may be designed to cause a phase
difference of substantially close to 0 or 120 degrees of light
coming from certain input and/or output ports. For example, PM
coupler 801 may have a first set of ports 821, 822, and 823 on one
side (first side) and a second set of ports 824, 825, and 826 on
the other side (second side). Being a bidirectional device, PM
coupler 801 may work either as a coupler or a splitter. Ports
821-826 may function or be used as input and/or output ports.
According to one embodiment, PM coupler 801 may be made of three ST
fibers fused in a middle section of single-mode fibers, such as PM
coupler 200 as being illustrated in FIG. 2. Ports 821, 825, and 826
may correspond to PM fiber segment of 221, 214, and 234, and ports
822, 823, and 824 may correspond to SM fiber segment of 211, 231,
and 224.
[0055] One or more optical signals launched into one or more ports
of one side of PM coupler 801 may be coupled, through propagating
inside thereof having polarization states being properly
maintained, to one or more ports of the other side of PM coupler
801. For example, a polarized optical input or optical signal
launched into port 821 of the first side may be coupled to ports
824, 825 and/or 826 of the second side of PM coupler 801. More
specifically, according to one embodiment, an optical signal or
light launched into port 821 may be coupled to ports 825 and 826 in
substantially equal amount and phase, which may have a phase
difference of close to, or substantially close to, 120 degrees from
that of port 821. In another embodiment, optical signals or lights
coupled into ports 825 and 826 from port 821 may be in
substantially equal amount but with a phase difference close to, or
substantially close to, 120 degrees with each other and with that
of port 821. In yet another embodiment, optical signals or lights
coupled to ports 825 and 826 may be in different amounts and with a
phase difference close to, or substantially close to, either 120
degrees or 0 degree, which in turn may have a phase difference of
close to, or substantially close to, 120 degrees from that of port
821.
[0056] In an opposite direction to that described above, polarized
optical inputs or optical signals launched into ports 825 and 826
of the second side may be coupled to ports 821, 822 and/or 823 of
the first side with their respective polarization states and
relative phase relationship among the ports, as described above,
being properly maintained.
[0057] According to one embodiment, end of fiber at port 824 of
second side may be treated with an anti-reflection coating material
and/or may be cut in an angle, such that back-reflections of light
or optical signal from the end of the fiber may be substantially
reduced, and/or preferably eliminated.
[0058] Electric current sensor 800 may also include a light source
820, being connected to port 821 of the first side of PM coupler
801; first and second photon-detectors 806 and 807, being connected
respectively to ports 822 and 823 of the same side; and a signal
processor 808, being connected to both first and second
photon-detectors 806 and 807. Electric current sensor 800 may
further include first and second polarizers 802 and 803, being
connected respectively to ports 825 and 826 of the second side of
PM coupler 801; first and second quarter-wave plates 804 and 805,
being connected respectively to first and second polarizers 802 and
803; and a current sensing fiber coil 810 that has two terminals
(ends) being connected respectively to first and second
quarter-wave plates 804 and 805. Connections between PM coupler
801, polarizers 802 and 803, quarter-wave plates 804 and 805, fiber
coil 810, and photon-detectors 806 and 807 may be through their
respective pigtail fibers, and may be made through, for example,
fusion splicing, optical connectors, and/or other currently
available or future developed techniques.
[0059] It shall be noted that configuration of electric current
sensor or current sensing device 800, according to embodiments of
the present invention, may not be limited in those aspects as being
demonstratively illustrated in FIG. 8 and various deviations and/or
variations from the configuration in FIG. 8 may be considered as
within the essence and spirit of the present invention. For
example, first and second polarizers 802 and 803 may be connected
in places other than between PM coupler 801 and quarter-wave plate
804 or quarter-wave plate 805. In one embodiment, first and second
polarizers 802 and 803 may be connected between photon-detector 806
(or photon-detector 807) and PM coupler 801, via port 822 (or port
823) at the first side of PM coupler 801. Also for example, a
polarizer (not shown) may be connected between light source 820 and
port 821 for controlling polarization state of light being launched
into port 821 of PM coupler 801. Further for example, an optical
isolator (not shown) may be used between light source 820 and port
821 for reducing or preferably eliminating light travelling
potentially backward from PM coupler 801 toward light source 820
which in certain instances may cause instability of operation of
light source 820.
[0060] During operation, light source 820 may launch an optical
signal into port 821 of PM coupler 801. The optical signal may
preferably be a linearly polarized, e.g., x-direction
(perpendicular to this paper) polarized light 10. However, the
present invention is not limited in this respect and other
polarized or non-polarized light may be used as well. In one
embodiment, a non-polarized light may become linearly polarized
after passing through a polarizer (not shown) inserted between
light source 820 and port 821 of PM coupler 801. Light 10 may
subsequently split, inside PM coupler 801 which functions as a
splitter in this instance, into two lights 11 and 21 of both
x-direction polarized and lights 11 and 21 may propagate toward
polarizers 802 and 803, via ports 825 and 826, respectively. In one
embodiment, lights 11 and 21 may have substantially same phases. In
another embodiment, lights 11 and 21 may have a phase difference
substantially close to 120 degrees.
[0061] Polarizer 802 may align linearly polarized light 11, or
convert a non-polarized light into linearly polarized light 11,
with a direction, with respect to quarter-wave plate 804, such that
after propagating through quarter-wave plate 804 linearly polarized
light 11 becomes a right circularly polarized light 12. For
example, polarization direction of light 11 may be aligned 45
degree relative to the main polarization axis or polarization
direction of quarter-wave plate 804. Similarly, polarizer 803 may
align linearly polarized light 21, or convert a non-polarized light
into linearly polarized light 21, with a direction, with respect to
quarter-wave plate 805, such that after propagating through
quarter-wave plate 805 linearly polarized light 21 becomes a left
circularly polarized light 22. For example, polarization direction
of light 21 may be aligned -45 degree relative to the main
polarization axis or polarization direction of quarter-wave plate
805.
[0062] Fiber coil 810 used for sensing electric current may be spun
around a medium such as a conductor or wire 821 that carries an
electric current under measurement, detection, or test. Electric
current carried by and flowing inside conductor 821 may create a
magnetic field along the optical path of fiber coil 810, causing
rotation of polarization direction of lights propagating therein,
which is known in the art as Faraday Effect. More specifically,
right circularly polarized light 12, after propagating through
fiber coil 810, may experience a first phase shift to become a
right circularly polarized light 13. Right circularly polarized
light 13 may then be converted, by quarter-wave plate 805, back
into an x-direction linearly polarized light 14 carrying a first
phase information which is directly related to the magnitude of
current inside conductor 821.
[0063] Similarly, left circularly polarized light 22, after
propagating through fiber coil 810 in a direction opposite to that
of light 12, may experience a second phase shift to become a left
circularly polarized light 23. The second phase shift may be
different from the first phase shift. Left circularly polarized
light 23 may then be converted, by quarter-wave plate 804, back
into an x-direction linearly polarized light 24 carrying a second
phase information which is also related to the magnitude of current
inside conductor 821.
[0064] Linearly polarized lights 14 and 24 may subsequently be
launched into PM coupler 801, via ports 826 and 825 respectively.
Since phase differences among different ports of PM coupler 801 are
substantially close to 120 degrees, as compared with 90 degrees of
a conventional 2.times.2 optical coupler, PM coupler 801 may create
coherent interference between linearly polarized lights 14 and 24
without the need for additional phase shift elements or devices as
required in a conventional fiber-based electric current sensor.
Coming out of PM coupler 801, a combination of lights 14 and 24 may
then propagate along ports 822/823 to photon-detectors 806/807,
wherein it is converted into a photocurrent. Electrical outputs of
photon-detectors 806/807 are connected to signal processor 808,
which receives photocurrents from photon-detectors 806 and 807,
processes information carried by the photocurrents, and determines
the amount of current carried inside by conductor 821.
[0065] FIG. 9 is a simplified block diagram illustration of an
all-fiber electric current sensor applying a 3-by-3
polarization-maintaining coupler according to another embodiment of
the present invention. Electric current sensor or current sensing
device 900 may include a 3-by-3 PM coupler 901, and has a first set
of ports 921, 922, and 923 on a first side and a second set of
ports 924, 925, and 926 on a second side. PM coupler 901 is a
bi-directional device working either as a splitter or a coupler,
and ports 921-926 may be used as input and/or output ports. PM
coupler 901 may be substantially the same as PM coupler 801 in FIG.
8.
[0066] Electric current sensor 900 may include a light source 920,
being connected to port 921 of the first side of PM coupler 901;
first and second photon-detectors 906 and 907, being connected
respectively to ports 922 and 923 of the same side; and a signal
processor 908, being connected to electrical outputs of first and
second photon-detectors 906 and 907. Electric current sensor 900
may also include first and second polarizers 902 and 903, being
connected respectively to ports 925 and 926 of the second side of
PM coupler 901; a polarization beam splitter (PBS) 904 connected to
both first and second polarizers 902 and 903; a quarter-wave plate
905 connected PBS 904; and a current sensing fiber coil 910. Fiber
coil 910 used for electric current sensing has one end connected to
quarter-wave plate 905, and another end being properly terminated
with a high-reflection coating material or a mirror such that
optical signals propagating towards the end may be substantially
reflected back into the fiber coil 910.
[0067] It shall be noted that configuration of current sensing
device 900 of present invention is not limited in aspects
demonstratively illustrated in FIG. 9, and various deviations
and/or variations from the configuration in FIG. 9 may be
considered as within the essence and spirit of present invention.
For example, first and second polarizers 902 and 903 may be
connected or placed in places other than between PM coupler 901 and
PBS 904. According to one embodiment, first and second polarizers
902 and 903 may be connected or inserted between photon-detectors
906 and 907 and PM coupler 901, via ports 922 and 923 at the first
side.
[0068] According to one embodiment of the present invention,
connection between polarizer 902 and one input/output branch of PBS
904 may be made through a 90-degree rotation of polarization axis
of their respective pigtail fibers. In other words, a polarization
axis of pigtail fiber of polarizer 902 may be made 90 degree, at
connection point 909, relative to that of pigtail fiber of PBS 904
such that, for example, an x-direction linearly polarized light
coming from polarizer 902 may be launched into PBS 904 as a
y-direction linearly polarized light.
[0069] During operation, light source 920 may launch an optical
signal into port 921 of PM coupler 901. The optical signal may be a
linearly polarized light, e.g., an x-direction polarized light 30.
However, the present invention is not limited in this respect and
other polarized or non-polarized light may be used as well. Light
30 may subsequently split inside PM coupler 901, which functions as
a splitter, into two lights 31 and 41 of both x-direction linearly
polarized propagating along ports 925 and 926 respectively.
Polarizer 902 connected to port 925 may ensure that light 31 from
port 925 is x-direction linearly polarized. Similarly, polarizer
903 may ensure that light 41 from port 926 is x-direction linearly
polarized.
[0070] At point 909, light 31 from port 925 is rotated 90-degree or
substantially close to 90-degree to become a y-direction linearly
polarized light and is launched into a first branch of PBS 904. On
the other hand, light 41 from port 926 remains as an x-direction
linearly polarized light and is launched into a second branch of
PBS 904 without polarization rotation or with a substantially close
to 0-degree polarization rotation. PBS 904 may combine two mutually
orthogonal linearly polarized lights and couple them into
quarter-wave plate 905. More specifically, PBS 904 may couple
x-direction linearly polarized light 41 and y-direction linearly
polarized light 31 into quarter-wave plate 905.
[0071] PBS 904 may be connected through its pigtail fiber to
quarter-wave plate 905 in such a way that quarter-wave plate 905
may convert a y-direction linearly polarized light, e.g. light 31,
into a left circularly polarized light 32, and convert an
x-direction linearly polarized, e.g. light 41, into a right
circularly polarized light 42. Left and right circularly polarized
lights 32 and 42 may propagate along fiber coil 910 respectively,
and may experience different amount of rotations of their
polarization states. The amount of rotation may be proportional to
the current inside conductor 921 but in different magnitude.
[0072] Upon reaching the other end or terminal 911 of fiber coil
910, left circularly polarized light 32 may be reflected back by
terminal 911 as light 33 to travel in opposite direction back
toward quarter-wave plate 905. Right circularly polarized light 42
may be reflected back by terminal 911 as light 43 to travel toward
quarter-wave plate 905 as well. Quarter-wave plate 905 may convert
light 33 into an x-direction linearly polarized light 34, and
convert light 43 into a y-direction linearly polarized light
44.
[0073] PBS 904 may direct x-direction linearly polarized light 34
to port 926, via polarizer 903, and y-direction linearly polarized
light 44 to port 325, via polarizer 302. In directing y-direction
linearly polarized light 44, light 44 is first converted into an
x-direction linearly polarized light, through the 90-degree
rotation of polarization axis of polarization-maintaining fiber at
point 909, and then the x-direction linearly polarized light is
launched into port 925.
[0074] Linearly polarized lights 34 and 44, which may carry
different phase information resulting from the rotation of
polarization direction caused by current inside conductor 921, may
be launched into 3-by-3 PM coupler 901. Since phase differences
among different ports of PM coupler 901 are substantially close to
120 degrees, as compared with 90 degrees of a conventional
2.times.2 optical coupler, PM coupler 901 may create coherent
interference between input lights 34 and 44. The interference may
subsequently be detected by photon-detectors 906 and/or 907 via
ports 922 and 923. Signal processor 908 may then receive
photocurrents produced by photon-detectors 906 and/or 907, process
the information carried by the photocurrents, and determine the
amount of current carried by conductor 921.
[0075] While certain features of the invention have been
illustrated and described herein, many modifications,
substitutions, changes, and equivalents will now occur to those of
ordinary skill in the art. It is, therefore, to be understood that
the appended claims are intended to cover all such modifications
and changes as fall within the spirit of the invention.
* * * * *